Liquid crystal display (LCD) panels are used in various applications ranging from portable electronic device to fixed location units, such as laptops, video cameras, mobile phones, PDAs, game machines, medical instruments, automobile navigation systems, and industrial machines. In LCD applications, usually backlighting is needed to illuminate the panel. Typically, LCD backlighting is used to provide high brightness, long lifetime and good uniformity features. There are many types of LCD backlighting sources, such as Electroluminescent Lamp (EL), Light Emitting Diode (LED), Cold Cathode Fluorescent Lamp (CCFL), Flat Fluorescent Lamp (FFL), External Electrode Fluorescent Lamp (EEFL), Hot Cathode Fluorescent Lamp (HCFL), and Carbon Nano Tube (CNT).
CCFL backlighting is commonly used in graphics and color displays, and is well suited for use in large and middle scale LCD panels. Moreover, CCFL can be used as the illumination source for LCD panels, and may be composed of a phosphor coated glass cylinder with cathodes at either end. Further, with the increasing size of LCD panels, e.g., in LCD televisions or large-size LCD monitors, backlighting systems may operate with multiple CCFLs to provide the necessary illumination.
A high voltage Direct Current/Alternating Current (DC/AC) converter (known as an inverter) is usually required to drive the CCFL. Most CCFL DC/AC converters may be formed as tuned switch circuits designed to produce an output AC power with a specific voltage and frequency. A typical CCFL inverter needs to output about 20˜80 kHz AC, with an operating voltage of about 400˜800 V RMS (Root Mean Square). Moreover, with the advent of large LCD panels where many CCFLs are needed, suitable approaches for driving multi-lamps are necessary. For multiple lamps, the DC/AC converter (inverter) drives multiple CCFLs usually in parallel. For example, referring to PRIOR ART FIG. 1, a conventional driving circuit 100 is illustrated. The driving circuit 100 is used to drive four CCFLs 142, 144, 146 and 148, and comprises a switch circuit 110 and two transformers 114 and 116. The transformers 114 and 116 have primary windings and secondary windings, respectively. The switch circuit 110 is used to convert an external DC electric power from a DC electric power source 112 into a first AC electric power, and to deliver the first AC electric power to the primary windings of the transformers 114 and 116. The secondary windings of the transformers 114 and 116 are coupled to the CCFLs 142 and 144 and the CCFLs 146 and 148, respectively, for energizing the CCFLs 142, 144, 146 and 148. Here, the transformers 114 and 116 are used to boost the first AC electric power with a relatively low voltage level to a second AC electric power with a high voltage level so as to meet the requirement for driving the CCFLs 142, 144, 146 and 148. Capacitors 118 and 120 are also coupled to the secondary windings of the transformers 114 and 116 in parallel, respectively.
Referring to PRIOR ART FIG. 2, another conventional driving circuit 200 is illustrated. The driving circuit 200 is used to drive a plurality of CCFLs 242, 244, and 246, and comprises a switch circuit 210 and a plurality of transformers 214, 216, and 218. The transformers 214, 216, and 218 have primary windings and secondary windings, respectively. The switch circuit 210 is coupled to the primary windings of the transformers 214, 216, and 218. The switch circuit 210 is used to convert an external DC electric power from a DC electric power source 212 into a first AC electric power, and to deliver the first AC electric power to the primary windings of the transformers 214, 216, and 218. The secondary windings of the transformers 214, 216, and 218 are coupled to the CCFLs 242, 244, and 246, respectively, for energizing the CCFLs 242, 244, and 246. Capacitors 222, 224 and 226 are also coupled to the secondary windings of the transformers 214, 216, and 218, respectively.
Those configurations have the well-known problem that the CCFL currents may not be balanced, owing to the lamp voltage variation and the load characteristics of the CCFL, as well as the differences in the CCFL impedances and temperature variation. The imbalance of the CCFL currents causes a reduced lifetime and non-uniformity of brightness.